1. Field of the Invention
This invention relates to a hydraulic braking system for a vehicle having an antiskid function whereby the hydraulic pressure at wheel cylinders thereof is modulated to prevent locking of the wheels as well as a holding function whereby the braking force is automatically maintained during vehicle stoppage, to make the vehicle immobile, and is automatically released upon operation by the operator to move the vehicle. More particularly, this invention relates to a hydraulic braking system having such dual functions, and further having a function of checking various components of the braking system and warning a vehicle operator with a visible or invisible alarm when any of the components have failed or are malfunctioning.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 59-149851, published Aug. 27, 1984, discloses a hydraulic braking system having a function of holding the vehicle immobile during stoppage (hereinafter referred to as holding function) in addition to an antiskid or antilock function. As illustrated in the figure thereof, the braking system includes a master cylinder 3 operated by a brake pedal 2 and connected through a hydraulic conduit 6 to wheel cylinders 5. The system is provided with a solenoid operated shutoff valve 7 located in the hydraulic conduit 6 and a solenoid operated second flow control valve 13 which is adapted to relieve the hydraulic pressure at the wheel cylinders when energized. In the antiskid control mode of the system, an antiskid control unit 9 controls the shutoff valve 7 and the control valve 13 to modulate the hydraulic pressure at wheel cylinders to avoid locking of the wheels and prevent the wheels from skidding. In the "holding" mode, upon sensing that the vehicle speed is zero, that the accelerator pedal is released, and that the clutch is disengaged, a cruise control unit 10 operates to close the shutoff valve 7, whereby the hydraulic pressure at the wheel cylinder is maintained, the brake remains applied, and the vehicle is kept immobile until the operator thereafter attempts to move the vehicle, for example, by engaging the clutch or pressing down the accelerator pedal.
In a hydraulic braking system of this nature having both a holding function and an antiskid control function, whenever the vehicle is stopped, the operator is able to determine whether or not the shutoff valve has functioned properly by checking if the vehicle is held immobile even though the brake pedal is released. However, the operator is unable to tell whether or not the antiskid control valve and associated hydraulic conduits are in a proper operable condition required for antiskid control.